The present exemplary embodiment relates generally to the printing arts. It finds particular application in conjunction with an apparatus and a method for identifying colors which are outside the color gamut available with a printing system. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications.
The range of colors, or gamut, as perceived by the human eye, captured on film, displayed on a computer monitor, or rendered by a printer varies significantly. Each has its own color space, a mathematical means of describing the colors which it can capture. L*, a*, b*, for example, are the independent space representations of the CIE (Commission Internationale de L'eclairage) for color standards which are often utilized in the functional modeling of these color demands. L* defines lightness, a* corresponds to the red/green value and b* denotes the amount of yellow/blue. RGB is an additive color space that combines red, green and blue light to create all other colors. RGB color spaces are used by monitors, digital cameras, and scanners. CMYK color, on the other hand, is a subtractive color space using cyan, magenta, yellow and black inks on paper to absorb red, green and blue light. The remaining reflected light is the color perceived by the viewer. Typically, the halftone density of a colorant to be used in rendering an image is specified by an 8 bit integer (a whole number between 0 and 255 in base 10 notation). The number 255 corresponds to the maximum density which can be achieved. A tone reproduction curve is used to determine which combination of available colorants (typically cyan, magenta, yellow, and black) will yield the desired colors specified.
While most color display monitors can display hundreds of thousands of colors using gray scale or continuous tones (contone), color marking devices, such as printers and copiers, usually have a significantly smaller number of producible colors. In halftone printing, for example, the image is made up of an array of pixels. The chromaticity of a given area is increased by turning on an increasing number of pixels of the colorant. The maximum density of a colorant is achieved, for a given area, when 100% of the pixels are turned on. A pixel is the smallest element of a printing system that can be independently controlled. Different printing devices have different color gamuts which are described in terms of the printer's output profile.
Customers often wish to make color copies using one printer, of hard copy documents which have been printed on another printer. For example, a customer may wish to make copies of an offset proof on a xerographic printer. One way to achieve a good match on a digital printer is to iterate through all the possible CMYK emulation modes on the printer's digital front end (DFE) and fine tune the color with custom Tone Reproduction Curves (TRCs). One problem with this approach is that there are often certain out of gamut colors, which are impossible for the printer to obtain. It may not be until some considerable trial and error with custom TRCs has been undertaken that a customer realizes that a color is out of their device's gamut and therefore unachievable. This can be frustrating for a customer since in addition to wastage of time, materials are consumed during the printing of test pages.